Lung transplantation is the definitive therapy for many end-stage pulmonary diseases and in many cases it is the only therapeutic option, despite having the highest mortality among all solid organ transplants. The fragility and the poor tolerance against ischemia of this organ is responsible for the fact that only 20% of the candidate lungs are currently being transplanted. The success of lung transplantation is limited by acute organ failure as well as chronic rejection against the transplant. Despite the improvement of surgical techniques and the development of better immunosuppressive drugs, short term airway complications taking place at the bronchial anastomosis (where the transplanted airways are surgically connected to the recipient' airways) continue to be a source of morbidity and mortality in those patients. Immediate ischemia of the donor bronchus and sacrifice of bronchial circulation during the surgical procedure have been recognized as the major risk factor for the development of airway complications.
The lung is unique among solid organ transplants in that it is not routinely reattached to the systemic circulation by bronchial arterial revascularization at the time of surgery. Blood supply to the airways in lung transplant recipients is therefore compromised with what blood flow is actually present presumably being provided by the deoxygenated pulmonary artery circulation. Therefore, from the onset, lung transplant airways have an impaired microcirculation due to the lack of a blood supply from the bronchial artery circulation, which results in relative airway tissue hypoxia. It has been previously demonstrated that the lack of bronchial arterial circulation in a lung transplant predisposes the transplanted airway to significant ischemia and hypoxia. It has also been shown that infectious agents can reside in the ischemic area, which includes the bronchial anastomosis of the transplant. Infection is one of the major causes of abnormal healing of the anastomosis as well as increased rate of acute rejection.
Several animal models, including the orthotopic tracheal transplant (OTT), heterotopic tracheal transplant, and orthotopic lung transplant models, have been used to study the pathology associated with human lung transplantation. The mouse OTT model has been shown to faithfully replicate the lymphocytic bronchitis seen following lung transplantation. The surgical anastomosis in the OTT model is similar to clinical transplantation in that it involves an end-to-end joining of donor with recipient airways. OTTs are therefore suitable for studying phenomena associated with clinical airway complications. It was previously shown that the airway microvascular circulation can be easily studied with the mouse OTT model and that the perfusion of the airway allograft can be used to assess the regeneration of the injured airway microvasculature, particularly at the anastomosis. The airway allograft is a free tissue, and there is no vascular perfusion prior to the formation of the microvascular anastomosis between the graft donor and the recipient. Therefore, earlier appearance of graft perfusion indicates an accelerated vascular anastomosis formation. Moreover, the OTT model is an ideal system to study airway microvascular repair and remodeling that occurs during alloimmune injury because of the well-organized planar anatomy of airway microvasculature. Using an OTT model, it was previously found that enhanced expression levels of hypoxia-inducible factor-1α (HIF-1α), the most important regulatory gene for hypoxic tissues, in airway grafts (by adenovirus-mediated gene therapy) promotes the recruitment of angiogenic cells, and prolongs tissue perfusion. It has also been shown that increased HIF-1α in the recipient cells promotes airway vascular anastomosis formation.
Ischemia is the principal factor that stimulates neovascularization, which is primarily regulated by HIF-1; this transcription factor consists of a constitutively expressed HIF-1β subunit and an oxygen-regulated HIF-1α subunit. In the presence of oxygen, two proline residues of HIF-1α are hydroxylated by the prolyl hydroxylase PHD2, facilitating von Hippel-Lindau tumor suppressor gene product (VHL) complex binding and HIF-1α degradation. In hypoxic conditions, PHD2 is inactive and HIF-1α is stabilized. HIF-1α then dimerizes with the β subunit, translocates to the nucleus, and induces gene transcription through binding to hypoxia response elements (HRE) of the oxygen-sensitive genes. HIF-1-mediated transcriptional responses orchestrate the expression of proangiogenic growth factors that facilitate angiogenesis by directly activating resident endothelial cells as well as recruiting circulating angiogenic cells.
Deferoxamine (DFO), deferasirox (DFX), and deferiprone (DFP) are all FDA-approved drugs for the treatment of iron overload conditions. DFO is a bacterial siderophore (N-[5-[[4-[5-[acetyl(hydroxy)amino]pentylamino]-4-oxobutanoyl]-hydroxyamino] pentyl]-N′-(5-aminopentyl)-1V-hydroxybutanediamide), DFX is a synthetic oral iron chelator (4-[(3Z,5E)-3,5-bis(6-oxocyclohexa-2,4-dien-1-ylidene)-1,2,4-triazolidin-1-yl]benzoic acid), DFP is an oral iron chelator (1,2-dimethyl-3-hydroxypyrid-4-one).
DFO, DFX and DFP have been extensively studied in various disease models. DFO can induce the transcriptional activity of HIF-1α in tumors. DFO stabilizes HIF-1α from degradation by inhibiting the activity of the PHDs through depletion of Fe2+. Both DFO and DFX were shown to promote β cell function through upregulation of HIF-1α. In a rat median nerve injury model, local administration of DFO-loaded lipid particle promoted end-to-end nerve reconstruction. Through stabilizing HIF-1α protein, DFO has recently been shown to potentiate the homing of mesenchymal stem cells to promote target tissue regeneration. In a mouse hind limb ischemia model, DFO was shown to promote vascular repair and relief tissue ischemia.
Drug-loaded nanoparticles have emerged as a promising strategy for efficient drug delivery for the treatment of a variety of diseases. Drugs encapsulated in nanoparticles may display increased availability due to higher specific surface area and biocompatibility of the formulated particles.
As the size of a particle decreases, the surface area to the volume ratio increases, leading to an increased dissolution velocity, as described by Noyes-Whitney equation. Additionally, the saturation solubility of a particle increases as the particle size decreases, as described by the Kelvin and Ostwald-Freundlich equation, particularly after the particle size falls below about 1 μm. These phenomena make a nanoparticle formulation a highly effective means to enhance mass transfer from the particle to the surrounding medium. By suspending a drug as nanoparticles, one can achieve a dose that is higher than that of a solution, which is thermodynamically limited by the aqueous solubility of drug.
There is a great clinical interest in formulations and methods to improve the success of solid organ transplants, particularly lung transplants. Current surgical procedure of lung transplantation and post-operative management cannot effectively prevent airway ischemia and associated airway complications. The present invention addresses the need to limit airway complications.